Field of the Invention
The present invention relates to a method for reselecting, by user equipment, a cell in a mobile communication system and user equipment using the same.
Related Art
In a mobile communication system, to support the mobility of user equipment (UE) is essential. To this end, UE continues to measure the quality of a serving cell that now provides a service and the quality of neighboring cells. UE having an RRC connection with a base station (BS) reports the results of measurement to a network at a proper time. The network provides the UE with optimal mobility through handover, etc. In contrast, UE (RRC IDLE UE) not having an RRC connection with a BS autonomously continues to perform cell reselection using information, such as the quality of a serving cell and the quality of neighboring cells.
Meanwhile, UE may move between cells within the same network (system) using the same radio access technology (RAT), but may also move between different networks using different RATs. In this case, RRC IDLE UE may perform inter-RAT cell reselection.
When UE makes an ATTACH request in order to access a first system using a first RAT, the access may be rejected due to several reasons. For example, if the UE has not joined the first system using the first RAT, although the first RAT is supported from a UE device aspect, access to the first system may be rejected.
In a situation, such as in the above example, when the first system transmits a rejection message, it may also transmit an indicator providing notification that all the tracking areas (TAs) of a first frequency in which the first system is operated has been barred with respect to the roaming of UE. In this case, the TA is an area including a plurality of cells and may be an area managed by a network in order to check the location of UE, that is, in the idle state. The UE may store a list of TAs in which roaming is barred.
Meanwhile, the UE that has received the indicator considers that roaming for all the TAs of the first system using the first frequency has been barred for a specific time, for example, for a maximum of 300 seconds and performs cell reselection under such a premise. Accordingly, in the cell reselection process, the UE does not select a cell of the first system using the first frequency.
In accordance with a current standard, however, after the specific time elapses, the UE considers a cell of the first frequency to be a candidate cell in a cell reselection process, reads system information from the first frequency, and performs measurement on the cell of the first system. Although the cell of the first system is unable to be selected in a cell reselection process because the cell is included in a TA list for which roaming has been barred, a process for performing measurement and reading system information is unnecessarily repeated.
There is a need for a method and apparatus capable of solving such a problem.